Trends in oil pollution in the southernmost (Belgian) part of the North Sea were analysed using a dataset of 37 years (1962-99) of annual national beached bird surveys conducted in February each year. The most abundant seabird groups represented in the beached birds were auks (31 %), gulls (28%), scoters (17%) and Kittiwake (9%). Oil rates of most bird species/taxa indicate a decline in oil pollution, though only Larus-gulls, Common Guillemot and Razorbill show significant reductions. The slope in the linear decreasing trend is steeper in inshore and midshore species, than in pelagic species. A power analysis of the results demonstrated that statistically significant trends in annual indices would be expected within 17 years for Razorbill, 29 years for Larus-gulls and 31 years for Common Guillemot. For other species/taxa, at least 50 years of surveying would be required. Long-term oil pollution monitoring in Belgium should be continued with a major focus on a set of abundant bird taxa, sensitive to oil-pollution and occurring in various marine habitats. Most appropriate for this purpose are grebes (inshore), Larus-gulls, Common Guillemot and Razorbill (midshore) and Kittiwake and Fulmar (offshore).